In recent years, since crude oil and natural gas resources in oil fields located on land or in so-called shallow seas having a water depth of up to around 500 meters are being depleted, development of offshore oil fields in so-called deep seas at a depth of 1,000-3,000 meters, for example, beneath the surface of the sea is being actively carried out. In deep-sea oil fields, it is necessary to transfer crude oil or natural gas from the wellhead of an oil well or natural gas well which is installed on the sea floor to a platform located on the surface using steel pipes referred to as flow lines or risers.
In steel pipes constituting flow lines or risers installed deep in the sea, a high internal fluid pressure to which the pressure of deep underground layers is added is applied to the interior of the pipes, and they also undergo the effects of water pressure of the deep sea when operation is stopped. In addition, steel pipes constituting risers are subjected to the effect of repeated strains due to waves. Furthermore, the sea water temperature deep in the sea falls to around 4° C.
Flow lines are steel pipes for transport which are installed along the contours of the ground or the sea floor. A riser is a steel pipe for transport which rises from the sea floor to a platform on the surface of the sea. When such pipes are used in deep sea oil fields, it is normally considered necessary for the wall thickness of such steel pipes to be at least 30 mm, and in actual practice, it is customary to use thick-walled pipes with a wall thickness of 40-50 mm. From this fact, it can be seen that flow lines and risers are members which are used in severe conditions.
The fluid produced in oil wells and gas wells in deep sea being developed in recent years often contain hydrogen sulfide, which is corrosive. In such environments, high strength steel undergoes hydrogen embrittlement referred to as sulfide stress cracking (SSC) and eventually undergoes failure. In the past, susceptibility to SSC was said to be highest at room temperature, so a corrosion resistance test for evaluating resistance to SSC was carried out in a room temperature environment. However, it has been found that in actuality, susceptibility to sulfide stress cracking is higher and cracking occurs more easily in a low-temperature environment of around 4° C. than at room temperature.
In a steel pipe for line pipe used as flow lines or risers, a material is desired which exhibits not only high strength and high toughness but also good corrosion resistance in a sulfide-containing environment. In this type of application, seamless steel pipe is used rather than welded pipe in order to achieve high reliability.
Corrosion resistance of steel for line pipe has hitherto placed stress on prevention of hydrogen induced cracking (HIC), i.e., on resistance to HIC. Among corrosion resistant steel pipes having a strength exceeding X80 which have been disclosed so far, there are many which emphasize HIC resistance. For example, JP 09-324216 A1, JP 09-324217 A1, and JP 11-189840 A1 disclose steels for line pipe of X80 grade having excellent HIC resistance. With these materials, HIC resistance is improved by controlling inclusions in the steel and increasing hardenability. However, with respect to resistance to SSC, there are no discussions therein concerning resistance to SSC at room temperature, not to mention resistance to SSC at low temperatures.
As described above, as the development of oil wells and gas wells in deep sea oil fields proceeds, the resistance to SSC of steel pipes for line pipes used as flow lines or risers is becoming important. In a low-temperature environment such as in deep sea oil or gas fields, susceptibility to SSC of high strength steels increases, and particularly with high strength steels having a yield strength (YS) of at least 80 ksi (551 MPa), susceptibility to SSC increases to an extent which cannot be ignored. Therefore, there is a demand for improvement in resistance to SSC in seamless steel pipes for line pipe made from high strength steels of at least X80.